Can my friend, who is a swordmage, and I, who am a battlemind, both have the same enemy marked at the same time?
I tried to read the rules and look up the answer but I am pretty new at D&D so I am having trouble.
No, a creature can only be marked by one opponent at a time.
From the Rules Compendium, p232 (emphasis mine):
A creature can only be marked by one person at a time. When a 2nd person marks the creature, the 1st person's mark is removed.